Different transmission media such as Cu two-wire line (Cu double lead line), coaxial cable, optical fibers (or radio as well) can be provided in the user line area of a telecommunication network for the connection of the individual subscriber stations, whereby Cu double lead line play a dominant part in existing telecommunication networks. The trend toward higher transmission rates (beginning in the later 1980's with the introduction of the integrated services digital network (ISDN) with a transmission capacity of up to 160 kbit/s on a 6 km long Cu double lead line) potentially leads to optical fiber links being brought up to a subscriber-proximate switching point in the user line area, what is referred to as the cable brancher (fiber to curb). In the branching cable area, that is, for the rest of the link between cable brancher and subscriber station, however, the Cu double lead line installed thereat has usually remained. In 90% of all cases, the distance still to be bridged with Cu two-wire lines amounts to less than 500 m in Germany. The corresponding distance is about 2-3 times greater in the United States.
What are referred to as HDSL (high bitrate digital subscriber line) systems and ADSL (asymmetric digital subscriber line) systems have been tested for Cu double lead lines since 1994. Respectively 2 Mbit/s can be transmitted over approximately 3 km with HDSL systems comprising two double leads. Two through six Mbit/s can be transmitted in downstream direction from the exchange to the subscriber station with ADSL systems comprising only one Cu double lead line, whereas only a low-rate return channel is available in upstream direction from the subscriber station to the exchange, whereby an analog signal channel (0.3 through 3.4 kHz) for conventional telephony (POTS--plain old telephone service) is provided from the outset in both directions.
Hybrid optical fiber/Cu double lead network architectures and transmission systems for the transmission of bit rates on the order of magnitude of about 12 through 50 Mbit/s are currently being considered for Cu double lead lines in American and European standardization commissions under the names High Speed Copper Drop, Broadband Digital Subscriber Line (BDSL) or Very High Bit Rate Digital Subscriber Line (VDSL), whereby both symmetrical (identical bit rates in both transmission directions) as well as asymmetrical systems (high bit rate downstream to the subscriber; lower bit rate upstream in the return channel) are being considered (ITG Trade Convention, "Zukunft der Kommunikationsnetze", Cologne Dec. 14, 1995, pages 87-96; ntz (1996) 2, 20-27). Here, too, the boundary condition of conventional telephony (POTS) is of significance.
A conventional telephony channel (POTS channel) is currently provided in parallel to the digital signal channels on subscriber lines only in the ADSL systems. The determinant requirement of enabling conventional telephony in parallel that determines the selection of the transmission system or, respectively, of the line code led to the fact in ADSL systems that only carrier systems (single carrier or multi-carrier systems) wherein the spectra of the carrier-modulated digital signal lie far above the conventional telephony channel were taken into consideration, so that a high-pass filtering has no influence thereon. The POTS channel is connected in and out at the subscriber and at the exchange side with frequency separating filters (splitters) having digital signal high-pass filters. The high-pass in the frequency separating filters at both ends of the channel can be a Butterworth high-pass of the 4th order with a limit frequency of 100 kHz in order to already achieve a blocking attenuation value of 60 dB at the charge tone (16 kHz) of the conventional telephone service.
Digital signal baseband systems, by contrast, are less suitable in the framework of ADSL systems because the signal-to-noise ratio at the receiver input is too greatly deteriorated due to the high-pass filtering at the subscriber side and exchange side, and the smoothing becomes too involved. Given, for example, a 2 Mbit/s signal with 4-PAM line code and Nyquist filtering, the frequency spectrum would thus be concentrated on the lower 500 kHz. A high-pass filtering with a high-pass of the 4th order having a limit frequency of 100 kHz at both ends of the channel would make this baseband transmission extremely involved in view of the smoothing and would lead to too great a deterioration of the signal-to-noise ratio at the receiver input since the especially beneficial lower frequency range of the channel (with low line attenuation and crosstalk interference) is precisely what cannot be utilized.
The same also applies to VDSL baseband systems. Here, too, the high-pass filtering with a high-pass of the 4th order having a limit frequency of 100 kHz at both ends of the channel would require an extremely high outlay for distortion correction increasing with the length of the channel pulse response and the height of the pulse trailer adjoining the principle pulse since the low high-pass limit frequency of 100 kHz for VDSL systems (with, for example, a baud rate of 12.5 Mbaud) leads to an extremely long channel pulse response and the high filter order required leads to large pulse trailers.